1. Field of the Invention
The present invention relates to fluorescent lamp ballasts and more particularly to apparatus for controlling the power level generated so the amount of produced illumination may be varied from full brightness down to no light at all.
2. Background Art
The control of the brightness of illuminating systems is well known and is relatively simple particularly for incandescent lights. On the other hand, fluorescent lights, while more economic to operate than incandescent lamps cannot be controlled by simple rheostat type devices. The control of dimming for fluorescent lamps typically employs special and much more costly and sophisticated ballasts in conjunction with other circuitry to achieve illumination intensity control.
A number of patents have been discovered which disclose the dimming of fluorescent lamps. These include the following:
U.S. Pat. No. 4,286,195 to Swinea Jr. which teaches a fluorescent lamp dimming circuit limited to DC fluorescent lighting circuits with a type using a battery voltage source and an inverter. The DC input is interrupted by chopping up the current, thus the controls power the ballast by pulsive energy.
U.S. Pat. No. 4,558,262 to Longenderfer et al teaches a simple dimmer for use with a bank of ballasts and located ahead of the ballast itself, but not located within the ballast as is taught in the present invention.
U.S. Pat. No. 4,568,870 to Chikuma teaches the use of a phase control device for regulating the illumination of a fluorescent lamp. Again the dimmer is located ahead of the ballast in series therewith and employs a conventional triac incandescent lamp dimmer circuit. It is doubtful whether the circuit will properly operate, since when dimming occurs the bimetal start switch would cause the lamp to flash on and off.
U.S. Pat. No. 4,792,729 to Peters teaches a load side phase control circuit in conjunction with the conventional fluorescent light ballast with an isolation transformer to achieve fluorescent lamp brightness control. A resistor and capacitor connected in series shunt the phase control circuit to maintain low level illumination when the phase control circuit is non-conducting.
U.S. Pat. No. 4,937,504 to Black, Jr. et al teaches a time delay circuit for start up of a ballast. It obviously is intended for use with a dimmer unit located ahead of the ballast but not built within the ballast itself as taught by the present invention.
U.S. Pat. No. 4,950,963 to Sievers again teaches a device located ahead of the ballast rather than within as in the present invention, but employs a triac dimmer with a timer to disable the dimmer on startup.
U.S. Pat. No. 4,994,718 to Gordin employs a capacitor in series with the ballast to reduce lamp voltage. By changing the value of the capacitor a certain amount of dimming is achieved. Again the capacitor is not included within the ballast circuitry but rather is located between the ballast and the lamp. It is doubtful whether this circuitry would be operable at low light levels.
U.S. Pat. No. 4,998,046 to Lester employs variable pulse modulation to control the dimming by including a separately powered constant voltage filament arrangement. In practice it becomes necessary to increase the filament voltage as the light level decreases to make up for the loss in arc current. The requirement for a constant filament voltage would probably be very expensive and of questionable operational value.
U.S. Pat. No. 5,004,959 to Nilssen teaches a modified ballast for gas discharge lamps wherein the lamp current control means are utilized to vary the current magnetic flux saturation and in response to the magnetic flux applied controls the frequency of the AC voltage. It indicates that the lamp operating current supplied is dependent on the frequency of the alternating current voltage. It is stated in this patent that the magnetic flux means is magnetically coupled with the charge and control means adapted to provide control of magnetic flux to the control means to permit control of the frequency of the AC voltage and thereby the magnitude of the lamp operating current.
A careful review of the teachings of this patent indicates that the collector current of the transistor varies with dimming and the width of the current pulse decreases with an increase in dimming, and that the frequency only increases slightly. It is not the frequency but rather the duration of the pulse that determines the light level of the associated fluorescent lamp. For example, at 100% light level it has been determined that the transistors are conductive 76% of the time and at 10% of the light level they are conductive only 64% of the time. This change in duty cycle of transistors doesn't explain why the lamp goes from 100% down to a very light output level. The reason appears that the lamp impedance increases as the energy to the lamp decreases. Thus, it has been determined that in the circuitry as shown the lamp resistance changes with dimming and will significantly affect the total circuits electrical characteristics. A minor decrease in pulse width on time of the transistors will result in a large change of lamp output.